Forming a silicon nitride (Si.sub.3 N.sub.4) layer is one of the most significant processes in the semiconductor industry. This is because a Si.sub.3 N.sub.4 layer is often used as a mask in etching SiO.sub.2. In depositing the Si.sub.3 N.sub.4 layer, the primary byproduct produced is NH.sub.4 Cl. The reaction generating Si.sub.3 N.sub.4 is shown below: EQU 3SiH.sub.2 Cl.sub.2 +7NH.sub.3 .fwdarw.Si.sub.3 N.sub.4 +3NH.sub.4 Cl+3HCl+6H.sub.2 ( 1)
The aforementioned reaction is usually performed in a reaction chamber. A device referred to as a CDG (capacitance diaphragm gauge) is usually used to monitor the pressure of the reaction process in the chamber. The construction of a prior art chamber and CDG is shown in FIG. 1. There are two pressure sensing devices in this system: a first pressure sensing device 24 and a second pressure sensing device 20. The purpose of the pressure sensing devices are to show the pressure in the chamber so that the operator can monitor the process. The second pressure sensing device 20 operates within the range of 0-1000 Torr pressure, and the first pressure sensing device 24 operates within the range of 0-10 Torr pressure. When the pressure in the sensing device is within the range of operation, the associated valve is opened. However, the valve is closed when the pressure in the pressure sensing device is beyond the range of operation.
Further, a pipe 12 has 3 branches, 12a, 12b, and 12c. Pipe 12a connects the chamber 10 and a main valve 14. Pipe 12b connects the first valve 26 and the second valve 22 to the pipe 12a . The pipe 12c connects the main valve 14 and the pump 18. When the main valve 14 is opened, the pressure in the pipe 12a, 12b and 12c are the same. When the main valve 14 is closed, the pressure in pipe 12a and 12b is typically 760 Torr. The pump 18 nominally operates to reduce the pressure to 0.005 Torr. Therefore, the pressure in the pipe 12c is also 0.005 Torr.
FIG. 3A shows in detail the pressure sensor 24. The pressure in the space between first valve 26 and the diaphragm 30 is typically set to be 10 Torr. The pressure in the chamber 10 is always the same as that in the pipe 12a and 12b.
Turning next to FIG. 2, curve 28 shows the pressure of the chamber and the CDG in an exemplary run. There are many processes included in a run. Referring to the horizontal axis of FIG. 2, from point a to point b, the chamber and the CDG are in "standby" mode. From point b to point c, the pressure of the chamber and the CDG is decreased until the pressure of the chamber and the CDG equals to 0.005 Torr. From point c to point d, the pressure of the chamber and the CDG are maintained at 0.005 Torr and a leakage check is processed. At this time, as shown in FIG. 3A, the valve 26 is opened and there is no NH.sub.4 Cl present in the first pressure sensing device 24.
From point e to f, the process in the chamber is performed, e.g., the deposition of the silicon nitride (Si.sub.3 N.sub.4) is done. The pressure of the aforementioned process is about at 0.12-0.3 Torr, so the pressure of the chamber and the CDG is maintained at about 0.12-0.3 Torr until the next process. At this time, as shown in FIG. 3B, the valve 26 is opened and the vapor phase NH.sub.4 Cl 31a is injected to the first pressure sensing device 24. The next step is to increase the pressure of the chamber and the CDG until the pressure equals to 760 Torr.
For the point after g, the chamber and the CDG are in the "standby" state again, and a run of the chamber and the CDG is thus finished. When the pressure exceeds 10 Torr, referring to FIG. 3C, the valve 26 is closed and the pressure in the first pressure sensing device 24 is maintained at 10 Torr. Thus, the NH.sub.4 Cl in vapor phase changes to the solid phase and deposits on the surface of the space between valve 26 and diaphragm 30 of the first pressure sensing device 24 including the surface of the diaphragm 30.
The aforementioned solid NH.sub.4 Cl not only affects the sensitivity of the diaphragm, but also may be a source of particle contamination in the next run in the chamber. For example, in a subsequent run, the solidified NH.sub.4 Cl 31b will be evaporated when the pressure is less then 10 Torr. As illustrated in FIG. 3D, when the leakage check or the reaction is processed under a pressure less than 10 Torr, the valve 26 is opened and the vapor phase NH.sub.4 Cl 31a passes through the valve 26 to the chamber 10. A contamination particle source is thus formed in the first pressure sensing device 24. The more times the CDG is used, the more amount of solid phase NH.sub.4 Cl is deposited in the first pressure sensing device 24.
According to the phase diagram shown in FIG. 4, unless the temperature of the first pressure sensing device 24 is always kept above 200.degree. C., the NH.sub.4 Cl in vapor phase will be deposited in the first pressure sensing device 24 when the pressure is about 10 Torr.
Furthermore, when the solid phase NH.sub.4 Cl is not thoroughly purged, the NH.sub.4 Cl in vapor phase will deposit on the surface of the first pressure sensing device 24 at 150.degree. C., even though the pressure is about 10 Torr. Unfortunately, it is hard to remove the solid NH.sub.4 Cl in the first pressure sensing device 24 because of the shape of the first pressure sensing device 24. If the first pressure sensing device 24 is not heated, the vapor phase NH.sub.4 Cl will be deposited in the first pressure sensing device 24, and the lifetime of CDG is about 1.5-2 months. If the first pressure sensing device 24 is heated to 150.degree. C. as used in the prior art, the vapor phase NH.sub.4 Cl will be deposited in the first pressure sensing device 24 also. The lifetime of the heated CDG mentioned above is about 1-1.5 years.
Because the vapor phase NH.sub.4 Cl is deposited on the surface of the diaphragm of the gauge in the prior art, the sensitivity of the gauge is degraded. The effect of the deposited residue is negligible when the gauge is used in a process of low pressure chemical vapor deposition fabricating a Si.sub.3 N.sub.4 layer of 1500-2000 angstroms in thickness. However, the effect of the NH.sub.4 Cl on the diaphragm is significant when the process is used to form a thin layer, such as a ONO layer of 50 angstroms in thickness.